The store gave its assessment of a picture linking gun sales to school preparedness. The photo, tweeted by @OMGItsBirdman Wednesday, caused a lot of controversy on the internet due to sensitivity about school shootings, and it has been retweeted more than 40,000 times.

“We have definite proof it was a prank,” Walmart spokesman Charles Crowson told The Associated Press on Friday. “This is a result of a collective effort by a number of associates who take things like this seriously.”

Crowson did not specify what proof Walmart had and he did not say that the sign was placed there for a staged photo or if an employee or customer moved the sign there or if it was placed there by mistake.